1. Field of the Invention
The present invention relates generally to wafer processing systems, and more particularly but not exclusively to electro-static chucks employed in such systems.
2. Description of the Background Art
Electrostatic chucks are employed to support wafers in a variety of wafer processing systems. In a high-density plasma chemical vapor deposition (HDP-CVD) system, for example, an electrostatic chuck clamps a wafer in place while a thin film is deposited on the wafer. An electrostatic chuck is so named because it clamps a wafer by electrostatic force.
FIG. 1A schematically show an example electrostatic chuck 110. Electrostatic chuck 110 includes a body 119, which includes contact points 112 for contacting a backside of a wafer 101. Only some of contact points 112 are labeled in FIG. 1A to avoid cluttering the figure. Contact points 112 may comprise raised areas to allow a cooling gas (e.g., helium) to be flowed under wafer 101 between contact points 112. Body 119 and its contact points 112 are made of a dielectric material, such as a ceramic. Wafer 101, body 119, and electrodes buried in body 119 (see electrodes 114 and 116 in FIG. 1C) form a capacitor that clamps wafer 101 in place when the electrodes are energized. Electrostatic chuck 110 may be a so-called Johnson-Rahbeck electrostatic chuck. A Johnson-Rahbeck electrostatic chuck includes a leaky dielectric body that allows charge to migrate from the electrodes to the surface of contact points 112.
FIG. 1B is a top sectional view schematically showing the general location of the bipolar electrodes of electrostatic chuck 110. FIG. 1B shows body 119 with its top surface removed. Referring to FIG. 1B, an electrode 114 and an electrode 116 are separated by an electrode gap 115. Electrode 114 may be energized to a voltage of one polarity, while electrode 116 may be energized to a voltage of an opposite polarity.
FIG. 1C is a side sectional view schematically showing electrodes 114 and 116 buried in body 119. Note that FIGS. 1A–1C are not drawn to scale. As shown in FIG. 1C, a rod 117 is coupled to electrode 114 for applying a voltage thereon. Another rod (not shown) is coupled to electrode 116. Electrode gap 115 prevents the two electrodes from shorting. When electrodes 114 and 116 are energized by applying voltages on them (e.g., +1000 volts on electrode 114 and −1000 volts on electrode 116), a voltage potential develops on the top surface of body 119 including on the tips of contact points 112 (i.e., 112-1, 112-2, . . . ). This results in an electrostatic force that clamps the wafer onto contact points 112.